Alisma subcordatum Raf.
Family: Alismataceae
American water-plantain, common water-plantain, southern water-plantain
Alisma subcordatum image
from USDA Plants Website  
Etymology: Alisma: Greek name for a water plant
Plants: erect or floating, perennial, semi-aquatic to aquatic, 4"-40" tall
Leaves: in basal rosette, with parallel veins, ovate to elliptical, long-stalked, smooth and firm, aquatic or terrestrial
Flowers: white to pink, 3-parted, 1/8" wide, petals only slightly longer than the sepals; inflorescence taller than the leaves and with 4 or more whorls of flowers per stalk; blooms May-Sept.
Fruits: dense ring of dry seeds; seed with 1 groove on the back
Habitat: shallow water; in muddy soil
Conservation Status: Native